This invention relates generally to the valve art and, more particularly, to fixed cone sleeve valves.
Fixed cone sleeve valves have been used for many years as free discharge valves. Fixed cone sleeve valves have been used primarily as control valves in reservoir discharge and turbine bypass systems because of their suitability for braking high pressure heads. A typical fixed cone sleeve valve consists of a cylindrical interior conduit that is surrounded by a thin, axially moveable sleeve. A conical valve seat with its apex pointing upstream is held in place downstream of the interior conduit by a series of radially extending vanes. The valve is operated by axially sliding the sleeve toward or away from the conical valve seat. The sleeve is moved axially up against the conical valve seat to close the valve, and is moved away from the conical valve seat to open the valve. The jet issuing from the valve resembles a hollow diverging cone that continues to spread out through the atmosphere. The energy of the jet is eventually dissipated by air friction as it is broken down into a fine spray. This helps to prevent erosion of downstream banks and plunge pools.
Fixed cone sleeve valves such as these are often used in reservoir discharge and turbine bypass systems because of their ability to operate under high pressure heads while still permitting accurate flow control. Hoods are sometimes installed on the downstream end of the valve to confine the expanding jet and reduce undesired spray. Although fixed cone sleeve valves are generally used for free discharge applications, they can also operate partially or fully submerged.
As explained below more fully, despite their popularity in free discharge and submerged discharge applications, fixed cone sleeve valves have not successfully been operated as in-line control valves where they would control liquid flow through a pipeline or series of conduits. This is because in the past when fixed cone sleeve valves were used in enclosed pipelines the discharge from the valve would create a significant amount of cavitation in the liquid flow downstream from the valve.
In-line control valves are often called upon to withstand large pressure head drops and to operate smoothly under high velocity and high flow conditions. However, if local pressures within the valve or jet drop below the vapor pressure of water, cavitation can result and cause serious performance problems. The mechanism for cavitation entails the formation of small vapor nuclei, their subsequent growth within low pressure regions of a flow, and their violent collapse as they enter regions of high pressure. The energy released by the collapse of the vapor cavities typically causes noise and vibration within a hydraulic system. Strong cavitation near valve or pipe boundaries can cause damage or even failure of system components. Regions of low pressure in valves typically occur immediately downstream from an abrupt change in a valve's interior geometry. The flow in these regions tends to be highly turbulent with numerous eddies.
In the case of a fixed cone sleeve valve, low pressure regions occur just downstream of the sleeve and the fixed cone. At higher flow rates, these regions are likely to initiate cavitation.
The phenomenon of cavitation in valves has been a constraint in their application for many years. In the case of free discharge valves, such as fixed cone sleeve valves, cavitation is not a problem because the surrounding air helps maintain atmospheric pressure levels in the jet, and because there is no hydraulic system downstream to damage. By contrast, in-line control valves are susceptible to cavitation due to the fact that they operate in a closed system that prevents natural aeration of the flow by the atmosphere.
For these reasons, and despite their ability to accurately control flow under high pressure heads, fixed cone sleeve valves have not successfully been operated as in-line control valves.